The present invention relates to an apparatus for preventing a block suspended by a boom cable from engaging a second block mounted on the outer end portion of the boom.
A crane is generally operated with a hook, grab, or similar device connected to a block suspended from by a boom cable. A winch is used to pay in or out the cable to raise or lower the hook etc. In addition, the crane may have a telescoping boom, and the boom may be pivotable from a position where it is nearly horizontal to one where it is nearly vertical. Any one of these operations can cause the block to come into engagement with the outer end portion of the boom from which it hangs. Such engagement can cause severe damage to the crane itself.
Prior art devices have attempted to keep the block a safe distance from the outer end portion of the boom. When there is contact between the tool and the end of the boom, it is called "two-blocking" because the block on the tool comes in contact with the block on the outer end portion of the boom. The present invention prevents this and is called an "anti-two-block" system.
There have been prior art "anti-two-block" systems for use with electric cranes. The patents issued to Rutan (U.S. Pat. No. 1,558,635) and Stoner (U.S. Pat. No. 1,834,985) show limit switches which disconnect electrical power to the hoist motor when "two-blocking" becomes imminent. The patent issued to White (U.S. Pat. No. Re. 22,525) discloses an electrical system which can compensate for the interaction between two crane functions (boom pivot and cable payout) and stop those functions when "two-blocking" is about to occur.
The prior art discloses systems which place control levers in a neutral position before the crane reaches an unsafe condition. One of these is disclosed in a patent to Spain (U.S. Pat. No. 3,792,780) in which the control levers are clamped in a neutral position by a fail-safe mechanism when the load on the crane exceeds a predetermined limit.